'And' and 'AND' or 'Or' or 'OR'

From decvax!harpo!npoiv!alice!research!dmr

Date: Fri Oct 22 01:04:10 1982

Subject: Operator precedence

Newsgroups: net.lang.c

The priorities of && || vs. == etc. came about in the following way. Early C had no separate
operators for & and && or | and ||. (Got that?) Instead it used the notion (inherited from B
and BCPL) of "truth-value context": where a Boolean value was expected, after "if" and
"while" and so forth, the & and | operators were interpreted as && and || are now; in
ordinary expressions, the bitwise interpretations were used. It worked out pretty well, but
was hard to explain. (There was the notion of "top-level operators" in a truth-value context.)

The precedence of & and | were as they are now. Primarily at the urging of Alan Snyder, the
&& and || operators were added. This successfully separated the concepts of bitwise
operations and short-circuit Boolean evaluation. However, I had cold feet about the
precedence problems. For example, there were lots of programs with things like if (a==b &
c==d) ...

In retrospect it would have been better to go ahead and change the precedence of & to
higher than ==, but it seemed safer just to split & and && without moving & past an
existing operator. (After all, we had several hundred kilobytes of source code, and maybe 3
installations....)

Dennis Ritchie

Handy Heuristic

Order of Evaluation

The moral of all this is that you should always put parentheses around an expression that
mixes Booleans, arithmetic, or bit-twiddling with anything else.

And remember that while precedence and associativity tell you what is grouped with what,